Generally, in the image processing context, an image frame to be displayed on a screen, for example, a television screen, is represented by a matrix structure of digital information representing a grid of pixels and each pixel has several color components assigned to it, for example, the brightness component Y and chrominance components Cr and Cb each having a level or amplitude for the pixel considered. Such a structure of pixels or “bit map” therefore corresponds, bit-for-bit or pixel-by-pixel, to the image (then called “raster” image) which has to be displayed on the screen. Also, generally, the structure of pixels is generally in the same format as that used for the storage in the video memory of the screen. And, the raster frame thus stored in the video memory will be read pixel-by-pixel on a line and line-by-line. This is then called “raster scan”.
Currently, the size of the frames used for high definition digital television, called HDTV, is a so-called “2k1k” size, that is to say comprising 1080 lines of 1920 pixels. Moreover, the frequency, that is to say the number of frames per second, is 60 Hz.
To transmit such an image signal delivered by the TV decoder, over the wired link linking this decoder to the television, it is necessary to perform a compression of the image signal delivered by the decoder. In effect, transmitting such an image signal without compression requires extremely high transverse speeds which are generally costly and create electromagnetic interferences.
This is why the signal delivered by the decoder is compressed.
Compression/decompression processing operations can also be necessary for the storage of the images in a memory internal or external to the decoder.
In effect, a video signal is generally received in an encoded format, for example, according to the H264 or HEVC standards, then is decoded into an image format of the RGB or YCbCr type that is more bulky in terms of memory space.
Now, various processing operations are usually applied to the decoded images. Also, between each processing operation, the images are stored, for example, in buffer memories, in the decoded format. In order to limit the capacity of the memories used, it is advantageous to perform a compression of the decoded images before storage in the memory followed by a decompression on reading in the memory before processing.
Such compressions/decompressions must not introduce image degradations.
Currently, a conventional compression of a video signal can be performed by applying a two-dimensional low-pass filter to the chrominance components of the image signal. However, even though the quality of the image finally displayed on the screen remains acceptable, high-frequency information of the image signal can be lost.
The French patent application filed in the name of the applicant under the number 1650814 proposed a method and a device for encoding/decoding a multidimensional signal, leading advantageously to a compression/decompression of the multidimensional signal, making it possible to preserve the spectral performance levels throughout the frequency band of the signal (absence of linear filter) while not being significantly affected by a Gaussian white noise.
In the video domain, this encoding/decoding method and device result advantageously in compression/decompression, making it possible to reduce the visible degradations of the image displayed, and make it possible to significantly increase the compression rate to achieve, as a minimum for example, a compression rate of 3×.
This prior art encoding method is based on a localized encoding using a separation of the information conveyed by the signal, for example, the color information of the image, into a pair of components, namely a gradient amplitude and a structure (local coordinate system). The gradient amplitude/structure pairing is computed sample-by-sample (for example, pixel-by-pixel) by searching for the pair having the minimum error in a zone of available candidate samples, for example, a zone located in the vicinity of the current sample.